FATIGUE OF GLASS FIBERS REINFORCED POLYAMIDES: MEAN STRESS EFFECTS AS A FUNCTION OF GLASS FIBERS ORIENTATION

Anisotropy has a major impact on fatigue properties of short glass fibres reinforced polyamides. The impact of load ratio on fatigue properties of these materials has been studied using jointly mechanical testing; digital image correlation and infrared thermography at a frequency of 3Hz. Materials tested contained 30% in weight of glass fibres, at 3 different glass fibres orientation. Haigh diagrams appear quite complex. Lines of constant life appear parallel in a wide range of load ratios, and their shape doesn't correspond to any classical mean stress correction. Material behaviour during fatigue tests also depends tremendously on load ratio. Hysteresis loops, strain accumulation, modulus evolution and self-heating have been followed during fatigue tests. Hysteresis loops appear very sensitive to stress amplitude, while strain accumulation depends highly on mean stress. Specimens submitted to negative load ratios show stronger modulus reduction than others, because of both higher damage level and larger thermal softening. Infrared thermography showed that a strong relationship between the load ratio of fatigue tests and the level of temperature achieved on specimens surface: for negative load ratios, at the same fatigue resistance, stress amplitude, hysteresis loops area and self-heating become more important. Correlations between mechanical values such as strain energy or hysteresis loops areas and fatigue resistance have been tested and are still investigated.